1. Field of the Invention
The present invention relates to a method of forming a tunnel oxide film of a superconducting tunnel junction type X-ray sensor element used in an X-ray detector for measuring an X-ray energy spectrum.
2. Description of the Prior Art
An X-ray detector of the related art using a superconducting tunnel junction, as shown in FIG. 2, has a tunnel oxide film (not shown in the drawings) formed on a surface of a first superconducting thin film 5 formed on a substrate 4, and has a second superconducting thin film 6 formed on the first superconducting thin film 5.
As a method of forming a tunnel oxide film of the related art, there is a method of introducing gas containing oxygen into a vacuum chamber immediately after forming the first superconducting thin film 5, and then oxidizing the first superconducting thin film 5 by leaving it as it is for a fixed period of time at close to room temperature.
It is also possible to carry out oxidation by generating plasma inside the gas containing oxygen.
However, with the structure of the related art, since the first superconducting thin film 5 is a sputtered film or a vapor deposited film, it has a maximum thickness in the order of 1 xcexcm. Because X-rays have a large permeating force, at this thickness a sufficient X-ray absorption efficiency is not obtained and as an X-ray detector there is insufficient detection efficiency.
For example, aluminum with a thickness of 1 xcexcm has an X-ray absorption efficiency of less than about 3% for the energy of fluorescent X-rays of iron atoms that are to be analyzed.
In trials, sufficient X-ray absorption efficiency has been confirmed in the related art by replacing the first superconducting thin film 5 with a bulk superconductor, but it has not been possible to form a high quality tunnel oxidation film on a bulk superconductor surface.
It is necessary to coat a tunnel oxidation film uniformly with a thickness of a few nm.
Uniformity can be evaluated by measuring surface roughness, and it is necessary for an average surface roughness value Ra to be in the same order as the film thickness, or less.
In order to address the above described problems, oxygen ions are irradiated to form a tunnel oxidation film after anodizing a surface of a bulk aluminum material.